In mobile notebook personal computers (PCs), there are needs for an improvement in the power supply efficiency for longer battery run time, and for reduction in the component mounting areas to reduce the sizes of printed circuit boards for reduction in size, weight and thickness of the PCs. However, the employed ICs of the PC, such as a CPU, a set of chips and a graphic chip, tend to use lower operation voltages to reduce their electric power consumptions, and require their respective different supply voltages. Hence the number of desired different supply voltages in the PC tends to increase.
Matsumura discloses in Japanese Patent Application Publication No. JP H 11-41825-A published on Feb. 12, 1999 describes a power supply switching device for switching power supplies depending on the load power consumption. This power supply switching device includes a battery to be incorporated into a portable device, and at least one or more constant voltage means. The power supply switching device further includes DC voltage converting means for generating a DC voltage lower than the battery voltage, switching means for switching between the battery voltage and the DC voltage generated by the DC voltage converting means to thereby supply the electric power to an input of the constant voltage means, and control means for controlling the switching means so that the power source should be switched to the DC voltage generated by the DC voltage converting means in a main driving state in which the power consumption is large, while the power source should be switched to the battery voltage in a standby state in which the power consumption is small.
A DC voltage conversion circuit which includes a plurality of DC-DC converters, each having a switching element, a diode, a capacitor and an inductor, is well known. DC-DC converters are disclosed by Maxim Integrated Products, Inc. in a document entitled “DC-DC Converter Tutorial”, Oct. 19, 2000 on a Web page at http://pdfserv.maxim-ic.com/en/an/AN710.pdf, and by ON Semiconductor in a document entitled “Understanding the Output Current Capability of DC-DC Buck Converters”, April, 2003-Rev.0 on a Web page at http://www. onsemi.com/pub/Collateral/AND8117-D.PDF, which are incorporated herein by reference.